The present invention relates to the use of plastic films for printing with organic inks in an inkjet process, the films being composed of PVC or of other polymers suitable for producing films, these polymers having been modified with additives in such a way as to make them suitable for inkjet printing with organic inks.
There is widespread use of PVC films or other polymeric film materials in place of paper in processes which use printing technology. The preferred method of printing here is the inkjet process, using ink solutions which are water-based. The inkjet process in particular permits digital printing, i.e. computer-controlled variation of the printed images, and thus permits image variability which is impossible using conventional screen printing or roller printing techniques. Since these aqueous inks are normally incompatible with the plastic film, the plastic films are provided with an outer layer (topcoat) which is normally composed of a binder compatible with the film and of relatively large amounts of hydrophilic fillers such as lime, silica gel, aluminum oxide, cellulose powder or the like, which permit penetration of the aqueous ink into the outer layer. In many cases, penetration of aqueous inks is further improved by adding detergents. An important application sector for films of this type is the production of labels, where the printable film has also been secured to a carrier film by way of a permanently flexible adhesive (cf., for example, U.S. Pat. No. 4,713,273). This process is, moreover, also used relatively widely to produce surfaces for advertisements, e.g. on city posters, on public transport, or on banners.
Since water-based inks are not weather-resistant long term and run when wetted, organic inks based on organic solvents have been developed for the inkjet process. These comprise, for example, ethyl acetate, methyl ethyl ketone, or cyclohexanone as organic solvent, PVC and acrylate resins as film formers and thickeners, and also dispersed or dissolved pigments. Unlike an aqueous ink, the inks based on organic solvents adhere directly to a plastic film, dry more quickly, and, due to their binder content, leave a weather- and water-resistant ink film on the plastic film surface. There are also known organic inks based on relatively high-boiling water-free solvents which comprise dispersed or dissolved dyes and binders and give weather-resistant images. These solvents are non-explosive, but they dry only slowly. Disadvantages of the organic inks are that the ink film is mainly present on the surface and is therefore susceptible to mechanical abrasion, and when dots of ink are applied they easily run, because drying is slow, and this militates against sharp profiles and the clean printing of different colors adjacent to one another.
It was therefore an object of the present invention to modify the films in such a way that colors applied using organic inks in an inkjet process penetrate the film surface and no longer run laterally. This is intended to cause the ink to dry more rapidly at the surface and become wipe-resistant, improving the resolution of the printed image and permitting an increase in printing speed.
Surprisingly, this object can be achieved by admixing sufficient amounts of cellulose esters with the synthetic polymer composition known per se and suitable for forming the film. A mixture of the synthetic polymer composition with other plastics or fillers proves to be ineffective.
The films of the invention can replace paper and topcoated films in inkjet printing processes used hitherto, and can also be used as a print feedstock in synthetic floor coverings or wall coverings, or else in weatherproof coverings for tents or trucks or advertising surfaces.
To produce the films which can be used according to the invention, a usual film composition in the form of a powder which comprises the plastic and usual auxiliaries such as plasticizers, fillers, stabilizers, pigments, etc. is mixed with an appropriate powder of the cellulose esters, plastified in a kneader, and, via a calender or by extrusion, processed to give films whose weight per unit area is from 50 to 100 g/m2. As an alternative, it is possible, of course, to laminate a layer of the above makeup with thickness from 10 to 50xcexc as an outer film to a base film made from other types of polymers and, for example, colored white using titanium dioxide, barium sulfate, or calcium carbonate, or to use a coextrusion process to produce the two layers together. It is further possible to produce films of this type in a known manner from a plastisol, i.e. a mixture made from synthetic polymer particles, fillers, and other auxiliaries, and converted to a paste in appropriate diluents and plasticizers, where appropriate amounts of cellulose esters have been incorporated into the plastisol, and to apply this as a layer by spreading or doctoring and to gel it at temperatures of 150-200xc2x0 C. to give a film. For reasons of stability, a stabilizing textile or a synthetic polymer film, or else a peelable silicone paper, is used as substrate. There are also known casting processes for producing films of this type, and these are likewise suitable.
The cellulose esters used according to the invention have substantively been esterified using acetyl, propionyl and/or butyryl groups, the residual content of free hydroxyl groups in the cellulose being small, from about 0.5 to 3%. Preference is given here to mixtures of acetyl groups with propionyl and/or butyryl groups, which give the plastic a low hardness. Particular preference is given to acetyl butyryl celluloses having, based on the mass of the cellulose ester, about 13% of acetyl and 38% of butyryl groups, alongside a residue of about 1.5% of hydroxyl groups. It is intended that the cellulose esters make up 5-50%, preferably 8-20%, of the film composition. Plastisols permit incorporation of up to 30% of cellulose esters. At higher levels, the composition becomes too stiff to process, and the tensile strength of the resultant films becomes too low. However, for the production of printable waterproofing materials, floor coverings or plastic wall coverings it is also possible to manufacture relatively thick layers of 200-500 xcexcm from plastisols which have an appropriate stable substrate.
For reasons of price, it is preferable to use PVC as film base material, this having been treated with about 10-30%, preferably 20%, of a usual plasticizer (such as dioctyl phthalate or diisodecyl phthalate, or else a usual polymeric plasticizer) based on the mass of PVC. However, it is also possible to use other synthetic polymer materials which can readily be molded to give films, such as polyethylene or polypropylene, polystyrene, polycarbonate, or else other polyesters.
Usual additives besides plasticizers are UV stabilizers, crosslinkers, fillers, pigments, etc.
Polyvinyl butyral, aluminum oxide, polymethyl acrylate, talc, EVA are also incorporated as film moderators into the PVC film. Unlike the cellulose esters, they do not give any improvement in the sharpness of profiles of thin lines of sprayed ink when comparison is made with a corresponding unmodified film, and they are therefore not suitable as additives which improve printing.
DE 43 04 308 A1 describes stiff films made from cycloolefin copolymers which can be used as a capacitor dielectric, for electrical insulation, as a packaging film, as a repro film, or as a viewing window for LCD cells. The films have high transparency, good dielectric properties, high softening points, and good impermeability to gas, but are unsuitable for use according to the invention as films for receiving organic inkjet inks. The possibility of mixing with a very wide variety of plastics and polymers, inter alia cellulose propionate, is mentioned speculatively, and it is possible for these to be compatible or incompatible and also, where appropriate, to form separate layers. Nothing is said as to the direction in which this admixture modifies properties.
DE 38 82 047 T2 relates to molded articles made from linear polyesters with a filler made from cellulose acetate microbeads, air-filled pore cavities being produced here by stretching the polyester phase around the cellulose acetate beads, which are incompatible with the polyester. Due to refraction, these air-filled cavities give the article an opaque appearance and permit liquids to penetrate these cavities, as into a sponge. Films produced in this way can be used as synthetic paper and can be inscribed using printing colors or aqueous ink. Products of this type are unsuitable for the inkjet process, since the capillary absorption of the ink leads to diffuse running of sprayed ink droplets.
DE 15 69 533 A describes a process for reducing the soilability of plasticized PVC surfaces, by adding a cellulose ester in the form of an organic solution to the PVC paste. The films produced in this way are said to be protected from soiling by dust, fats, oils, and not, and to have high resistance to ballpoint pen inks and normal writing inks. Use in an inkjet process therefore appears impossible.